A coding of moving pictures, in general, divides a picture into a certain size of blocks and performs intra picture prediction and inter picture prediction for each block. It then applies orthogonal transformation, for example, discrete cosine transform or the like for each block of the smallest unit of a division (i.e. 4×4 pixels) so as to perform coding using variable length coding based on run level coding for coefficients showing spatial frequency components gained by orthogonal transformation.
The variable length coding assigns variable length code to values of the coefficients contained in the block to which orthogonal transformation is applied (level) as well as to numbers consisting of a series of a coefficient 0 (run). In this case, a table which corresponds the values with variable length code is called a VLC table. Under the conventional method, only one table is prepared as a VLC table respectively for intra prediction coding and inter prediction coding (reference to ISO/IEC 14496-2:1999(E) Information technology—coding of audio-visual objects Part 2: Visual (1999-12-01) P. 119 7.4.1 Variable length decoding).
Under the variable length coding method explained in the existing technique, only one table is prepared as a VLC table respectively for intra prediction coding and inter prediction coding. Therefore, it contains a problem that coding efficiency differs greatly depending on a quality of a current picture to be coded.
In order to solve this problem, a method of preparing a plurality of tables so as to refer to them by switching between them according to the number of coefficients other than 0 contained in a current block to which orthogonal transformation is applied is conceivable. For realizing this, it is necessary to perform coding by applying variable length coding for the numbers of the coefficients other than 0, however, the coding method and the decoding method are not yet established.